This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
In order for wireless systems to provide wireless coverage for elongated areas in a cost-effective manner, leaky transmission lines such as leaky coaxial cables have been proposed and widely deployed as a supplement to the basic wireless network infrastructure.
In contrast with an ordinary transmission line whose outer conductor is specially designed for minimizing electromagnetic radiation, a leaky transmission line has openings deliberately arranged on its outer conductor. Via these openings, electromagnetic waves may leak out of the leaky transmission line to provide wireless coverage for an elongated area along the leaky transmission line.
As elongated areas in the real world (such as expressways, railways and tunnels, etc.) are typically much longer than any single leaky transmission line and signals undergo significant attenuation while travelling along leaky transmission lines, amplifiers are often used with leaky transmission lines to construct a transmission network covering an entire elongated area.
Two typical topologies of networks consisted of leaky transmission lines and amplifiers are respectively illustrated in FIGS. 1-2 and will be described in the following.
Referring to FIG. 1, a so-called cascade structure is illustrated, which includes a signal source 110, two leaky transmission lines 140 and 141 coupled via a amplifier 121 and a terminator 130. The signal source 110 provides communication signals compliant with any known or heretofore unknown wireless networks and couples the communication signals into the leaky transmission line 140 in a wired manner. After transmitted through the leaky transmission line 140, the communication signals are amplified at the amplifier 120 and then fed into the leaky transmission line 141. After transmitted through the leaky transmission line 141, the communication signals are terminated at the terminator 130.
As a straightforward extension of the cascade structure illustrated in FIG. 1, a cascade of three or more leaky transmission lines can be formed to connect the signal source 110 to the terminator 130, with more than one amplifiers concatenated therebetween. To prevent the quality of transmitted communication signals from being significantly deteriorated by noise accumulation and nonlinear product due to concatenation of too many amplifiers, the number of concatenated amplifiers is normally limited to be less than 3.
FIG. 2 illustrates a so-called T structure, which includes a signal source 110, an amplifier 120, two terminals 130 and 131, two leaky transmission lines 140 and 141, a divider 150 and a long distance transmission line 160. The signal source 110 supplies communication signals onto the long distance transmission line 160. The amplifier 120 receives and amplifies the communication signals carried on the long distance transmission line 160, and supplies the amplified communication signals into both of the leaky transmission lines 140 and 141 via the divider 150. After transmitted through the leaky transmission lines 140 and 141, the amplified communication signals are terminated at the terminators 130 and 131, respectively.
By using an optical fiber as the long distance transmission line 160 and including the amplifier 120 in an optical repeater, the T structure as shown in FIG. 2 is suitable to be used in connection with optical fiber distribution systems.
In practical implementations, the cascade structure and the T structure can be combined as needed to form more complex topologies. One example of combinations of the cascade structure and the T structure is presented in FIG. 3. As illustrated, a signal source 110, a long distance transmission line 160, an amplifier 120, a divider 150 and two leaky transmission lines 141 and 142 constitute a T structure. From the leaky transmission line 141, a cascade structure consisted of an amplifier 121, a leaky transmission line 140 and a terminator 130 is extended as one branch of the T structure. From the leaky transmission line 142, a cascade structure comprising the leaky transmission line 142, at least one amplifier 122, at least one leaky transmission line 143 and a terminator 131 is extended as the other branch of the T structure.
In existing communication systems having leaky transmission lines and amplifiers to extend wireless coverage, every amplifier is configured to always amplify its incoming communication signals to a normal power level Pnormal, so that electromagnetic waves leaked from leaky transmission lines disposed in correspondence with sections of an elongated area are strong enough to provide constant wireless coverage for the entire area. As such, whenever a vehicle (such as an automobile, a train, etc.) carrying a user equipment (UE) travels along whichever section of the area, the UE can be provided with wireless service having a satisfactory quality of service (QoS).
Sometimes, however, there may be no vehicle present in some or all sections of the area. In this scenario, it is a waste of power to keep all amplifiers amplifying their incoming communication signals to a normal power level Pnormal and thus provide constant wireless coverage for the entire area.